1. Field of the Invention
The present invention concerns a radiation transducer of the type in which a luminophore layer is applied on a substrate. The invention furthermore concerns a method to produce a radiation transducer of this type.
2. Description of the Prior Art
A radiation transducer of the above type is used in a digital x-ray detector (flat panel detector) in combination with an active matrix that is sub-divided into a number of pixel readout units with photodiodes. The incident x-ray radiation is initially converted in the luminophore layer (scintillator layer) of the radiation transducer into visible light that is transducer by the photodiodes into electrical charge and stored with spatial resolution. This conversion, known as an indirect conversion, is described in the article by M. Spahn et al., “Flachbilddetektoren in der Röntgendiagnostik” (“Flat panel detectors in x-ray diagnostics”) in “Der Radiologe 43 (2003)”, Pages 340 through 350, for example.
Typical luminophore layers are composed of CsI:Tl, CsI:Na, NaI:Tl or similar materials that contain alkali halogenides. CsI is particularly well suited as a scintillator material since it can be grown in needle shapes. A good spatial resolution of the x-ray image is thereby obtained in spite of a greater layer thicknesses (which ensures an optimal absorption of the x-ray radiation). The good spatial resolution results from what is known as the “optical wave guide effect” that is achieved by the air gaps between the CsI needles.
Due to their content of alkali halogenides, the scintillator materials are at least slightly hygroscopic and must be sufficiently protected from damaging environmental influences (humidity excessively high temperature). For example, under the influence of temperature, humidity and air the CsI needles can merge into one another (degradation). The important air gap parameter is severely reduced. As a result of this the spatial resolution is reduced (measurement variable MTF—Modulation Transfer Function).
To avoid such degradation, the scintillator layers are protected with a suitable encapsulation (“packaging”), as this is described in DE 10 2006 022 138 A1, for example. A “yellowing” of the luminophore layer occurs in the course of this packaging process, which leads to an impairment of the light yield.
The detectors mentioned above (CCD chip, photodiodes etc.) normally have the greatest sensitivity in the spectral range from 500 to 520 nm. In contrast to this, most scintillators (radiation transducers) based on alkali halogenides (for example CsI:Tl) and described in the literature have a radiation maximum of 540 to 560 nm. As described in WO 2007/012564 A1, the scintillator layer of the radiation transducer is modified based on this such that the shortwave branch of the emission spectrum (50% values) of the radiation transducer is shifted from 480 nm to a smaller 440 nm. The maximum of the emission is shifted from 540 nm to 560 nm to a value around 500 nm. For this purpose, the content of Tl in the scintillator layer is 200 ppm to 2,000 ppm.
The applied packaging process leads to a “yellowing” (blue-green light is more strongly absorbed) of the converter layer, accompanied by a step-by-step shift of the shortwave branch of the emission spectrum back from 430 nm to 460 nm (50% values). This leads to the described impairment of the light yield since the maximum of the emission spectrum of the luminophore layer becomes separated step-by-step from the maximum sensitivity of the detector.